<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let num = 10;
        console.log(typeof num);    // number
        let str = '一';
        console.log(typeof str); // string
        let flag = false;
        console.log(typeof flag); // boolean
        let un;
        console.log(typeof un); // undefined
        let obj = null;
        console.log(typeof obj); // object
    </script>
</body>
</html>